Steve
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Post by Steve on Nov 10, 2020 13:20:15 GMT 12
This one will be a work in progress... made by Radio Warehouse Ltd in Christchurch in 1946 for Hays department store.
I have some photos of what the dial should look like from a set in far better nick than mine, and so I'm creating a new one from scratch
Slowly going through the process of recreating the dial from a de-skewed and stretched photo of the dial... here it is part-way through the process.
There are a few guide lines on the image that I use to line things up that will get removed later on - and the outer grey rectangle represents the visible area through the cabinet. I'm about 2 hours into the dial so far, and I expect another 2-3 should see it finished.
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Steve
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Post by Steve on Nov 10, 2020 13:25:55 GMT 12
Here is how I know the date of the set
Radio Warehouse, who made Austin sets as well, used to date-stamp their chassis' in the same way librarians used to stamp the slip in the back of library books. You can see the date here beside the speaker - 23 Feb 1946. The speaker code ends in L45. L = 12th letter, for 12th month = December, 45 for 1945. So that lines up.
This one is missing a transformer, and is a bit of a mess underneath - but its nothing too bad I don't think, so I should be able to sort it out reasonably easily.
Lineup is (from memory) 6K8, 6SK7, 6B8, 6F6, 6X5 (rectifier is missing, but the wiring lines up for a 6X5GT).
Should be fun
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Post by Richard on Nov 11, 2020 16:24:13 GMT 12
I wonder why it used an 6SK7 instead of 6K7 ?, when it used a 6K8. one less grid cap perhaps?
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Steve
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Post by Steve on Nov 11, 2020 17:40:16 GMT 12
Given it was just post-WWII and production would have only just kicked back in I can only assume it was a case of 'use what they could get', and probably also a case of 'use whatever costs the least'. Stage one, dial artwork complete - printed on card and fitted behind the glass to check alignment. The next step will be to reverse the artwork and laser-etch it into acrylic, then paint-fill the etching with red and green as appropriate. I will need to do quite a bit of electrical work to this chassis to be able to feed a signal in and check the alignment - so it might be like this for a while. Although I do have a piece of scrap acrylic I can probably use to see how it looks. Its definitely a bit lighter than the one I got the images for the artwork from looks. I also made the station ID's a bit bolder than they are on the original, and to my eye they clutter the dial a bit, so I'll 'debold' them at some point before the final dial is made. From this: To this:
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Steve
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Post by Steve on Nov 13, 2020 11:57:02 GMT 12
So because I was really busy and, therefor, looking for ways to procrastinate... This is a scan of the original dial, provided by a collector who has one of these. It has an interesting yellowy background colour that I originally thought was nicotine / dirt but I don't think so - I think it might be part of the original design - something I don't recall seeing before. I'll leave it out for now - but I could always do something with some very light window tint film or something similar. This is me discovering that our laser cutter will engrave (and actually cut!) glass. But its not pretty, and too much power causes shattering. I only had a small piece to play with but I suspect if I could do multiple passes at a low power I could probably get a decent depth, but this one was too shallow and so it didn't take much ink - it just doesn't look good enough to use - but it was an interesting experiment and I may get some glass to try different methods and see if I can find something that works. And so here is the glass experiment beside the acrylic that I am more used to using... which came out pretty good I think.
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Steve
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Post by Steve on Nov 13, 2020 12:15:55 GMT 12
And I drew out the schematic as well as I could from what is left under the chassis - this is what I've got. I'll redraw it properly at some point, but for now I have some questions. Why is there a cap bridging the first IF (it looks original, or at least the solder looks undisturbed for a long time)? And what value is it (my cap meter says 30nF but that seems weird - it has 220A on it, 220pF?). Tone control on the plate of the first audio... normally on the output valve? And its a 500k pot - should it be lower? 6B8 screen and plate directly connected? Is this normal? 50k resistor bypassing the 1st IF primary to feed HT to the 6K8 plate - why? Thoughts? I'll try and find a transformer for it in my spares today and see about powering it up...
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Post by Richard on Nov 14, 2020 7:16:24 GMT 12
If you were going to broaden the IF response, I would have thought a 30pf cap? not 30uF,also the 50k across the primary of the IF, to broaden the IF response? though I've read of using 100K resistors for that purpose. The screen of the 6B8 shouldn't be connected to the plate, even if you were wanting maximum gain. the Data sheet on the 6B8 says maximum screen voltage 125 Volts. I've seen the tone control circuit on the plate of the 1st AF before, with the pot being 250K.
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Steve
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Post by Steve on Nov 14, 2020 10:12:36 GMT 12
I'll take that cap out of circuit and measure it properly... I wonder why they needed to widen it - sloppy match between tuning components? Its not something I've come across before... but I haven't done too much with these small manufacturer sets. I've fitted a transformer (ignore the wiring, its tacked in for now just to make sure things are going to work), and recapped most of it. I fired it up last night on the dim bulb and things started off ok - the initial flare of light, then dimming out... then it started to climb in brightness beyond what I would have expected - then it started flaring and flashing, like there was some arcing going on. I didn't see anything, but on closer inspection the first IF coil is rattling around loose in the can - so possibly the primary side shorting in the can? I also used a random 6X5 from the drawer so I'll check to make sure I don't have a shorting one. I'll look more closely when I have a chance - I'm packing up the chainsaws to head off and sort out some trees for a mate
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Post by Richard on Nov 14, 2020 15:53:49 GMT 12
My guess is, they have used the capacitor and resistor across the IF coils is to widen the IF bandwidth, which will improve the audio frequency response. Of course the down side is the radio wont be as selective when tuning.
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Steve
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Post by Steve on Nov 15, 2020 17:14:42 GMT 12
Turns out a radio serviceman somewhere in the past used the 220pF (about 245pf measured) cap and 50k resistor to resolve a completely munted 1st IF - they were bypassing it. The arcing / flaring behaviour from the dim bulb that I mentioned was a dodgy 6X5GT... I replaced it and that went away. One side was open due to a broken wire as seen above, and the other was open for reasons that were not immediately apparent, the coil former was also broken and floating loose inside the can. A Philips IF can from a model 540 parts chassis from about the same year gave up its internals (including Hunts trimmers, because all good radios need Hunts caps in them ) to resolve that problem. So I've given it a quick alignment and it seems to be running about as good as any 5-valve set does out here in the country. Its probably as far as I'll go with it electrically, aside from a new power cord (although I might take a look at why the screen and plate on the 6B8 are connected). As the weather warms up I might refinish the cabinet but for now I think I'm going to put the dial on, refit it to the cabinet, take a few photos, and call it done Photos to follow
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Steve
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Post by Steve on Nov 15, 2020 17:59:37 GMT 12
Quick update - the 6B8 plate and screen were getting about 30V... so the #1 rule, "Though shalt check voltages" was ignored because I hadn't even measured that up until now... there was a 100pF Aerovox cap between the plate and cathode, seen here (also I've circled the jumper from the plate to the screen): Its the only obvious point of leakage - and so I've replaced it and also removed the tied between the plate and screen and instead fitted a 1.8M resistor from HT to the screen. I now have 90V (from memory) on the plate, and 25V on the screen... and noticeably more volume. Cheers, Steve
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Post by DHockey on Nov 16, 2020 8:21:10 GMT 12
Nice work Steve, I’m impressed by your dial making abilities! Those old radio servicemen were pretty clever at getting a set going again despite things like lack of parts and lack of money from the customer!
Regarding the tied plate and screen of the 6B8, maybe the set originally had a 6Q7 which needed replacement and a 6B8 was all that was available?
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Steve
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Post by Steve on Nov 24, 2020 18:12:11 GMT 12
So here it is. In the photo the dial looks clearer with the dark background - and I think this is how it was originally. In real life it looks a little too dark to me, so I tried it with the grey background and it looks better in real life, but washed out in the photo. I'm still undecided on what to do there. It needs some tidying up, but is running well enough now. Of course the last thing I did before it went in the cabinet was tighten up the nut on the new (old) switched tone control, and it sheared off the threaded sleeve... sigh... so I had to find another one. That was mildly annoying, but its sorted now. It must have been pre-stressed because I hardly put any weight on it.
So it needs new grill cloth and a wee tidy up in the cabinet - also knobs but I can live with the ones its wearing for now - I have about 10 of them, and only 3 with grub screws that aren't frozen solid with rust...
for now, its done (ish)
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Post by Richard on Nov 25, 2020 15:47:39 GMT 12
excellent reproduction dial Steve
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Steve
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Post by Steve on Dec 6, 2020 13:09:41 GMT 12
I wasn't going to do anything to the cabinet... but it was just sitting there taunting me... so here it is drying in the sun after scraping, sanding and an oxalic acid scrub-down... Next step, final sanding and shellac. I'm not sure if I'll oil it, wax it or lacquer it. Its particularly windy here at the moment so spraying is not going to happen - so while I would prefer to lacquer it, a wax finish might be the most achievable option.
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